This invention relates to a low carbon, Ni-Cr-Mo system cast iron roll for use in the hot rolling of a steel, and more particularly to a roll of the type describes for use in hot finishing-rolling.
Table 1 shows the chemical composition of a conventional cast iron roll for use in hot finishing rolling of steels.
Table 1 __________________________________________________________________________ Chemical composition (%) C Si Mn P S Ni Cr Mo Conventional 3.1 0.5 0.5 0.020 0.020 4.0 0.5 0.2 cast iron roll -3.3 -1.2 -0.8 below below -5.0 -2.0 -0.3 __________________________________________________________________________
For producing the above listed roll, iron melt having the above listed chemical composition is cast in a metal mold, and then the surface of casting is cooled rapidly, thereby obtaining a fine structure improving wear resistance, while the internal portion and the neck portion of the roll is slowly cooled to thereby produce graphite in a sufficient amount, for the purpose of obtaining softness and withstanding a high stress incurred to the rolling operation. The mechanical properties of a roll are shown in Table 2.
Table 2 ______________________________________ Roll internal Roll surface portion and neck portions ______________________________________ Tensile 30 - 35 kg/mm.sup.2 20 - 22 kg/mm.sup.2 strength Hardness 70 - 80 35 - 40 (Shore) ______________________________________
This type of a cast iron roll undergoes surface roughness and wear within several hours after the beginning of rolling operation, and then becomes no longer usable, so that the roll should be replaced. The operating hours required for replacement corresponds to about 10% of the total operating hours, presenting the most serious problem in the rolling, from viewpoints of efficiency and quality of products.
The outermost surface of the roll in hot rolling reaches temperatures ranging from 600.degree. to 800.degree.C.
Meanwhile, wear incurred to the roll is not a simple wear. More particularly, minute cracks develop in cementite on the surface of a roll and scales are peeled therefrom, so that scales cause friction heat to thereby soften the base iron, resulting in seizure. This then further roughens the surface of a roll and causes wear.
Cementite of 40 to 50%, in volume, is present in the surface of the conventional cast iron roll containing from 3.1 to 3.3% carbon, and is brittle and low in thermal conductivity, thus tending to produce cracking due to change in rolling heat and rolling load, while cementite is insusceptible to friction and to lowering in hardness at an elevated temperature as well as seizure to a hot strip. However, this is not true in the case of hot rolling, as has been described earlier.
On the other hand, the base iron contains from 4.0 to 5.0% Ni in addition to Cr and Mo. This gives Shore hardness of 70 to 80 at a room temperature, while it gives low hardness at an elevated temperature. Particularly at a temperature of 600.degree.C and over, it is rapidly softened, causing surface roughness and wear. However, in a portion deep about 100 microns from the surface of the roll, the temperature is steeply lowered, so that there arises a danger of the mechanical properties of a roll being impaired. In other words, a roll during rolling operation is subjected to flattening under a high rolling load, so that there may not be achieved accurate rolling, because of defects such as bite and buckling which causes hexagonal cracks in the roll surface. In addition, there may arise rupture of the opposite end portions of a roll due to dent and bending which are caused by inclusion of forgein matter such as hard scales and due to impingement of the edge face of a cooled slab.
As has been described, there are confronted many problems with rolls. Thus, it has been long desired to have a roll which meets the following requirements, in combination:
1. Cementite in the top or outermost surface of the roll shall not be subjected to cracking and the resulting peeling.
2. The base iron shall not be softened.
3. There shall be achieved high mechanical properties.
However, an existing cast iron roll fails to meet the aforesaid requirements, because of the high content of carbon, and the fact that nickel is primarily responsible for increasing hardness.